The production of benzene by hydrodealkylation of toluene or of hydrocarbons of the polysubstituted methyl-aromatic type has been practiced for many years. The operation takes place under hydrogen pressure at temperatures of between 530.degree. and 720.degree. C.; the reaction may be activated either by heat alone or by heat and a catalyst. When activation is thermal, the operation takes place between 620.degree. and 720.degree. C., and when activation is catalytic, temperatures of between 530.degree. and 650.degree. C. are preferably used, depending upon the activity of the catalyst employed. When the charge to be treated consists mainly of aromatic hydrocarbons as are the charges emanating from an extraction unit or a high-duty reforming plant, the operation is found to be a quite straightforward one, but it becomes complicated when the charge to be treated contains a certain proportion of saturated hydrocarbons of the naphthenic or paraffinic type (normal or iso paraffinic hydrocarbons), such as the charges emanating from a low-duty reforming plant or from a steam-cracking installation or a unit for the distillation of the light condensates of coke-ovens. In this last-mentioned case, the presence of paraffinic and naphthenic products, the content of which may vary from 2 to 20% by weight, increases the exothermicity of the operation, since the saturated products are converted mainly into methane, and furthermore, there will also be a tendency to accelerate the deposition of coke in the reaction vessel and especially when the reaction vessel is filled with catalyst; therefore, in the art as hitherto practised, it has been preferred to use thermal activation without using a catalyst even if the operation has to be carried out at a higher temperature with all the inconveniences that can be caused by the use of elevated temperatures as regards the nature of the materials used for the construction of the ovens or the reaction vessel.